Hadron production in lepton-proton scattering (semi-inclusive deep inelastic scattering, SIDIS) probes the structure of hadrons at a higher level of detail than fully inclusive processes. A wealth of SIDIS data is available especially from fixed-target experiments. Here we review our calculation for the NNLO corrections to the full set of polarized and unpolarized SIDIS coefficient functions and present some selected analytical expressions. Our results enable for the first time a fully consistent treatment of hadron fragmentation processes in polarized and unpolarized DIS at NNLO and provide the basis for studies of hadron structure, hadron fragmentation and identified particle cross sections at colliders.